Don't Panic if 0 Peers are Left (prysmaticlabs#4594)

* log error
* Merge branch 'master' into dontReturnError
* return blocks
* change back
* Merge branch 'dontReturnError' of https://github.com/prysmaticlabs/geth-sharding into dontReturnError
* use a more static finalized epoch
* Update beacon-chain/sync/initial-sync/round_robin.go

Co-Authored-By: Raul Jordan <raul@prysmaticlabs.com>
* Merge refs/heads/master into dontReturnError
* jim's review
* Merge branch 'dontReturnError' of https://github.com/prysmaticlabs/geth-sharding into dontReturnError
* Update beacon-chain/sync/initial-sync/round_robin.go

beacon-chain/sync/initial-sync/round_robin.go

@@ -9,6 +9,7 @@ import (
 	"sync/atomic"
 	"time"
 
+	"github.com/prysmaticlabs/prysm/beacon-chain/flags"
 	"github.com/libp2p/go-libp2p-core/peer"
 	"github.com/paulbellamy/ratecounter"
 	"github.com/pkg/errors"
@@ -44,15 +45,20 @@ func (s *Service) roundRobinSync(genesis time.Time) error {
 	counter := ratecounter.NewRateCounter(counterSeconds * time.Second)
 	randGenerator := rand.New(rand.NewSource(time.Now().Unix()))
 	var lastEmptyRequests int
+	highestFinalizedSlot := helpers.StartSlot(s.highestFinalizedEpoch() + 1)
 	// Step 1 - Sync to end of finalized epoch.
-	for s.chain.HeadSlot() < helpers.StartSlot(s.highestFinalizedEpoch()+1) {
+	for s.chain.HeadSlot() < highestFinalizedSlot {
 		root, finalizedEpoch, peers := s.p2p.Peers().BestFinalized(params.BeaconConfig().MaxPeersToSync, helpers.SlotToEpoch(s.chain.HeadSlot()))
 		if len(peers) == 0 {
 			log.Warn("No peers; waiting for reconnect")
 			time.Sleep(refreshTime)
 			continue
 		}
 
+		if len(peers) >= flags.Get().MinimumSyncPeers {
+			highestFinalizedSlot = helpers.StartSlot(finalizedEpoch + 1)
+		}
+
 		// shuffle peers to prevent a bad peer from
 		// stalling sync with invalid blocks
 		randGenerator.Shuffle(len(peers), func(i, j int) {
@@ -82,8 +88,8 @@ func (s *Service) roundRobinSync(genesis time.Time) error {
 			}
 
 			// Short circuit start far exceeding the highest finalized epoch in some infinite loop.
-			if start > helpers.StartSlot(s.highestFinalizedEpoch()+1) {
-				return nil, errors.Errorf("attempted to ask for a start slot of %d which is greater than the next highest epoch of %d", start, s.highestFinalizedEpoch()+1)
+			if start > highestFinalizedSlot {
+				return nil, errors.Errorf("attempted to ask for a start slot of %d which is greater than the next highest slot of %d", start, highestFinalizedSlot)
 			}
 
 			atomic.AddInt32(&p2pRequestCount, int32(len(peers)))
@@ -177,7 +183,8 @@ func (s *Service) roundRobinSync(genesis time.Time) error {
 			0,                    // remainder
 		)
 		if err != nil {
-			return err
+			log.WithError(err).Error("Round robing sync request failed")
+			continue
 		}
 
 		// Since the block responses were appended to the list, we must sort them in order to